A technique for epitaxially growing on a single FIG. 1 shows examples of hetero epitaxial growth. In crystalline substrate a material different from the substrate is known as the hetero epitaxial technique. general, if the crystal structure of a substrate and that of a film to be formed on the substrate are similar and their lattice constants are close, the film can be epitaxially grown on the substrate. When, however, any difference in lattice constant exists, a stress is generated in the film formed by epitaxial growth. When the film thickness exceeds the critical film thickness, lattice shift (misfit dislocation) occurs to relax the stress.
For example, when SiGe is to be epitaxially grown on Si, as the lattice constant of Si is 5.43 Å and that of SiGe (Ge concentration: 20%) is 5.47 Å, a lattice mismatch of about 0.8% is present. As SiGe grows on the Si lattice, the lattice constant in the direction parallel to the lattice plane becomes equal to that of Si in the initial stage of the growth, and SiGe is deposited in a compressed state. When epitaxial growth is continued until the film thickness exceeds the critical film thickness, misfit dislocations occur. The critical film thickness differs depending on the epitaxial growth conditions and is approximately 50 nm to 300 nm. When epitaxial growth is further continued, the misfit dislocations gradually increase. When the film thickness reaches about 2 μm, occurrence of dislocations mostly ends. In this state, SiGe is free from strain because of a “shift” accompanying the misfit dislocations and accordingly does not receive any stress, so that its lattice constant is 5.47 Å. A misfit dislocation causes a threading dislocation the end of which reaches the surface of the epitaxial growth film. Accordingly, many threading dislocations (e.g., about 1×107/cm2) are present in the surface of the epitaxial growth film having a thickness of 2 μm. Defects such as threading dislocations impart adverse effects to devices, e.g., characteristic degradation, characteristic variations, and short service life.
U.S. Pat. No. 6,503,773 discloses a technique for changing the Ge concentration in the direction of depth and accordingly relaxing the stress stepwise to divert the defect growing direction to a direction parallel to the structure surface, thereby decreasing the density of threading dislocations in the uppermost surface. According to this technique, the density of threading dislocations in the surface can be decreased.
Japanese Patent Laid-Open No. 2002-217116 discloses a technique for performing etching during a hetero epitaxial step to form a recess at a position where a threading dislocation is expected to occur, and resuming the hetero epitaxial step after that to grow crystals while leaving the recess as a space. The space blocks the threading dislocation from propagating to the upper layer.
With the technique described in U.S. Pat. No. 6,503,773, to cause stepwise concentration change, SiGe must be inevitably deposited thick. Then, the throughput decreases, and the manufacturing cost increases. If SiGe is deposited thick while including a defect, surface microstructures increase inevitably. Hence, after the epitaxial step, planarization such as a CMP (Chemical Mechanical Polishing) process becomes necessary.
With the technique described in Japanese Patent Laid-Open No. 2002-217116, the recess for forming the space that blocks propagation of the threading dislocation is formed by using the threading dislocation. If a threading dislocation should occur after etching, it occurs in a portion where no space is present, and accordingly propagation of the threading dislocation is not blocked by a space. To prevent occurrence of a threading dislocation after etching, the hetero epitaxial growth film must be completely relaxed before performing the etching. This complete relax is indispensable in commercialization. To completely relax the stress, a sufficiently thick hetero epitaxial growth film must be deposited before performing the etching.
More specifically, with the techniques described in U.S. Pat. No. 6,503,773 and Japanese Patent Laid-Open No. 2002-217116, to sufficiently relax the stress in the hetero epitaxial growth layer, the hetero epitaxial growth layer must be sufficiently thick. This limits the throughput and increases the manufacturing cost.